(1) Field of the Invention
The present invention relates to a polycrystalline transparent spinel sintered body and a method of producing the same.
(2) Description of the Prior Art
A transparent spinel sintered body consisting mainly of aluminum oxide and magnesium oxide has a cubic system crystal structure, and is free from double refraction. Therefore, the transparent spinel sintered body has an excellent light transmission in visible spectrum and infrared ray.
As the method of producing polycrystalline transparent spinel sintered body, the following methods have hitherto been known.
(1) A stoichiometric mixture of Al.sub.2 O.sub.3 and MgO is hot pressed.
(2) A stoichiometric powdery mixture of Al.sub.2 O.sub.3 and MgO is fired together with calcium oxide (CaO) or magnesium oxide (MgO).
(3) A powdery mixture of Al.sub.2 O.sub.3 and a stoichiometrically excess amount of MgO for spinel is fired together with more than 0.2% by weight of LiF.
(4) Alumina single crystal (sapphire) or polycrystalline high-density alumina sintered body is contacted with MgO vapor to form a spinel.
However, these methods have the following drawbacks. That is, in the method of hot pressing a stoichiometric mixture of Al.sub.2 O.sub.3 and MgO of the above item (1), a die made of carbon is used, and therefore the resulting sintered body is always a dark sintered body having a simple shape, and moreover the sintered body is expensive due to its low productivity. In the method of firing a stoichiometric powdery mixture of Al.sub.2 O.sub.3 and MgO together with CaO or MgO of the above item (2), and in the method of firing a powdery mixture of Al.sub.2 O.sub.3 and a stoichiometrically excess amount of MgO for spinel together with more than 0.2% by weight of LiF of the above item (3), a relatively large amount of additive must be used, and therefore the resulting sintered body is poor in the purity and is poor in the light transmittance. In the method of contacting alumina single crystal or polycrystalline alumina sintered body with MgO vapor of the above item (4), it is very difficult to control the amount of MgO vapor, and hence the production of a homogeneous sintered body is difficult.